Watermarking method for motion picture image sequence

ABSTRACT

A method is provided for applying a watermark pattern along a length of photosensitive medium, the photosensitive medium having a frame pitch in the length direction that is the sum of an image frame height plus an interframe distance. The method exposes watermark tiles contiguously along the length of the photosensitive medium,. wherein each watermark tile has a height dimension that is an integral divisor of the frame pitch.

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to the following commonly assigned disclosure: “Methodand Apparatus for Watermarking Film” to Roddy et al. U.S. Ser. No.10/364,488, filed Feb. 11, 2003.

FIELD OF THE INVENTION

The invention relates generally to the field of image watermarkapplication and more particularly relates to a watermarking method formotion picture film.

BACKGROUND OF THE INVENTION

An unfortunate result of technological advances in image capture andreproduction is illegal copying and distribution of image content, inviolation of copyright. One solution for counteracting illegal copyingactivity is the use of image watermarking as a forensic tool.Sophisticated watermarking techniques enable identifying information tobe encoded within an image. A watermark can be embedded in the imagebeneath the threshold of visibility to a viewer, yet be detectable underimage scanning and analysis. As just a few examples: U.S. Pat. No.6,239,818 (Yoda), discloses embedding a pattern in a color print andadjusting cyan, magenta, yellow, black (CMYK) values such that theembedded data matches the color of the surround when viewed under astandard illuminant; commonly assigned U.S. Pat. No. 5,752,152 (Gasperet al.), discloses a pattern of microdots, less than 300 μm in diameter,for marking a photographic print that is subject to copyright.

Illegal copying is a particular concern to motion picture studios anddistributors, representing a noticeable source of lost revenue.Watermarking of motion picture images would enable the source of anillegal copy to be tracked and would thus provide a deterrent to thisactivity. Watermarking techniques for still images and prints, however,may not be well-suited to motion picture film media. An encoded patternthat might not be easily visible within the single image of a printcould become visible and annoying if it appears in a sequence of imageframes. Moreover, a motion picture watermark must be detectable from acopy, such as a videotape copy, that is typically captured in a temporalsequence that varies from the motion picture projection rate and withvarying image resolution, lighting, and filtering. For these and relatedreasons, motion picture watermarking typically requires a special set oftechniques beyond those normally applied for still images.

A number of watermarking methods for motion images have been describedin prior art patents and technical literature. Included are methods thatapply a spatial-domain or frequency-domain watermark. In eitherapproach, many techniques make use of a pseudo-random noise (PN)sequence in the watermark generation and extraction processes. The PNsequence serves as a carrier signal, which is modulated by the originalmessage data, resulting in dispersed message data (that is, thewatermark) that is distributed across a number of pixels in the image. Asecret key (termed a “seed value”) is commonly used in generating the PNsequence, and knowledge of this key is required to extract the watermarkand the associated original message data.

Among prior art patents that address watermarking methods for motionpicture image content are U.S. Pat. No. 5,809,139, issued Sep. 15, 1998to Girod et al., entitled “Watermarking Method and Apparatus forCompressed Digital Video”; U.S. Pat. No. 5,901,178, issued May 4, 1999to Lee et al., entitled “Post-Compression Hidden Data Transport forVideo”; and U.S. Pat. No. 5,991,426, issued Nov. 23, 1999 to Cox et al.,entitled “Field-Based Watermark Insertion and Detection”. However, themethods disclosed in these patents can be applied only to a digitalvideo data stream and are not directly applicable for watermarkingmotion picture film.

U.S. Pat. No. 6,026,193, issued Feb. 15, 2000 to Rhoads, entitled “VideoSteganography”, discloses the basic concept of using multiplewatermarked frames from an image sequence to extract the watermark witha higher degree of confidence than would be obtained with only a singleframe. U.S. Pat. No. 6,449,379 issued Sep. 10, 2002 to Rhoads, entitled“Video steganography methods avoiding introduction of fixed patternnoise” proposes an improvement to this scheme by changing the PN carrierfrom frame to frame, for example.

Another approach to applying a watermark to an image sequence is to usea three-dimensional watermark pattern. An example of such a method canbe found in a paper by J. Lubin et al, “Robust, content-dependent,high-fidelity watermark for tracking in digital cinema,” in Security andWatermarking of Multimedia Contents V, Proc. SPIE, Vol. 5020, Jan. 24,2003. This paper discusses a method for embedding, into successive imageframes, a watermark containing low frequency content in both the spatialand temporal dimensions. The method described by Lubin et al. mayprovide a temporally distributed watermark that is relatively robust.However, this method requires temporal synchronization in order torecover or decode the watermark. That is, some mechanism must beprovided that allows indexing of each image frame with a referenceframe; a sampling of successive image frames must include this referencein order to allow synchronization of watermarked frames and subsequentdecoding. Another limitation is that knowledge of the image content isrequired for embedding a three-dimensional watermark using this scheme.

The prior art methods cited above provide some amount of watermarkingcapability, but are not well-suited for printing watermarks onto motionpicture film media. This is because each of these methods requires thatframe boundaries for each image be known prior to printing the watermarkonto the motion picture film medium. That is, the precise location ofimage frames on the motion picture medium must be known.

For photosensitive media in general, it is known that a watermarkencoding can be added to the image frame at the time of printing.However, it is also possible to expose a watermark at other times duringprocessing of the photosensitive medium. For example, as is disclosed inU.S. Patent Application 2003/0012569 entitled “Pre-Exposure of EmulsionMedia with a Steganographic Pattern” by Lowe et al., a latent image canbe exposed onto the “raw” photosensitive medium itself, at the time ofmanufacture. Then, when the medium is exposed to form the image, theimage frame is effectively overlaid onto the watermark pattern. Such amethod is also disclosed in U.S. Pat. No. 6,438,231, entitled “EmulsionFilm Media Employing Steganography” to Rhoads. The Rhoads '231 patentdiscloses this type of pre-exposure of the watermark onto the filmemulsion within the frame area of negative film, for example.

It can be appreciated that watermark pre-exposure would have advantagesfor marking motion picture film at the time of manufacture or prior toexposure with image content. A length of motion picture film could bepre-exposed with unique identifying information, encoded in latentfashion, that could be used for forensic tracking of an illegal copymade from this same length of film. However, prior art watermarkingtechniques proposed for photosensitive media in general fall short ofwhat is needed for motion picture watermarking. In particular, prior arttechniques are not well-adapted for applying a watermark pattern duringfilm manufacture or at any other time prior to exposure of the film withimage content. These prior art solutions prove unsatisfactory due, inlarge part, to these practical considerations:

-   -   (i) how motion picture film is imaged and projected in practice;        and,    -   (ii) how the motion picture film surface area or “real-estate”        is employed.

Referring to FIG. 1, there is shown a plan view of a typical motionpicture print film 10 that is used commercially. A first problem ((i)above) relates to the placement of frames 12 along the length directionL. Frames 12 are dimensioned and spaced with a frame pitch F accordingto well-established standards followed, for each film type, throughoutthe motion picture industry. In addition, perforations 14 are made inthe film, sized and spaced apart according to rigid standards. For 35 mmfilm, for example, manufacturers comply with the Society of MotionPicture and Television Engineers (SMPTE) Standard ANSI/SMPTE 139-1996entitled “SMPTE Standard for Motion-Picture Film (35-mm) Perforated KS”.Perforations 14 are formed at the time of film manufacture. Frames 12,however, are not formed until printing at the print lab. Thus, the exactlocations of frames 12 along print film 10 are not yet defined at thetime of manufacture. The starting position of each frame 12 may be knownrelative to an index perforation 14; however, it is not known whichperforation 14 is used as a reference index until the photosensitivemedium is exposed. Even though frames 12 correlate spatially withperforations 14, so that each frame 12 plus interframe space 16corresponds to an exact number of perforations (typically 4 perforationsper frame), the film manufacturer cannot know beforehand where eachframe 12 will lie. Thus, unlike the frame-by-frame placement suggestedfor the broad range of photosensitive media in the Rhoads '231disclosure, any practical exposure watermarking scheme for motionpicture film must apply a contiguous pattern along the full length offilm. This means that the watermark pattern will be exposed onto bothimage frames 12 and interframe spacing 16 areas.

This first concern, then, relates to dimensional characteristics of thewatermark. Tiling, in which multiple versions of a watermark arerepeated contiguously within the image frame, has been widely recognizedas a useful method for encoding the watermark. For example, commonlyassigned U.S. Pat. No. 6,044,156 entitled “Method for Generating anImproved Carrier for use in an Image Data Embedding Application” toHonsinger et al. discloses an image watermark scheme using one or moretiles. FIG. 2 shows an example in which multiple watermark tiles 20 arecontiguously arranged within image frame 12. (For illustration, only aportion of image frame 12 is shown covered in FIG. 2; in actualpractice, the complete area of image frame 12 would have watermark tiles20.) Watermark tiles 20 can be arranged contiguously in both length Land width W dimensions, extending widthwise between perforations 14 oreven between opposite edges of the medium.

Regardless of watermark tile 20 dimensions, the watermark can bedetected only when it occurs within image frame 12. However, unlesswatermark tile 20 is positioned length-wise at the same relativeposition within successive frames 12, the watermark will effectively“walk” up the displayed motion picture frame when the motion pictureimage is displayed. While this beat-frequency effect might not bedetectable over portions of an image sequence that are themselvesvisually busy, this “walking” effect could be noticeable and visuallyobjectionable at some frequencies and under some imaging conditions.More importantly, changing the vertical position of watermark tiles 20from one frame 12 to the next complicates the task of detecting thewatermark. This first consideration, then, directly affects therobustness of the watermark. The disclosure of commonly assignedcopending application “Method and Apparatus for Watermarking Film” toRoddy et al., U.S. Ser. No. 10/364,488, cited above, recognizes thisvertical alignment problem and proposes, as a corrective strategy,sizing the watermark tile height to span a single pitch perforation.While this provides a workable solution in view of problem (i) notedabove, it constrains the tile height dimensions to a single perforationpitch, where perforation pitch can be defined as the center-to-centerdistance between perforations. It can be appreciated that a moreflexible solution would eliminate this constraint and allow moreflexible sizing of watermark tile height, based on film type, forexample.

The second problem ((ii) above) relates to the use of the width W ofprint film 10. As FIG. 1 shows, the real-estate of the surface of printfilm 10 is occupied not only by the sequence of frames 12, but also byvarious audio tracks. An analog sound track 18 is printed between theside edge of frames 12 and perforations 14. A DTS (Digital TheaterSystems) soundtrack 26 is encoded between frames 12 and analog soundtrack 18. A Dolby digital sound track 22 uses areas interspersed betweenperforations 14, repeated on both sides. Another digital sound track 24,conventionally the standard SDDS (Sony Dynamic Digital Sound) track isencoded along edges of print film 10. Digital sound tracks 22 and 24 aretypically duplicated on both sides of print film 10 as indicated bydigital sound tracks 22′ and 24′. For considerations of watermarkapplication, it is significant to observe that analog sound track 18 anddigital sound tracks 22, 24, and 26 are encoded onto print film 10 usingexposure to light, in much the same way as frames 12 are exposed. Forthis reason, any imperfection in imaging quality of print film 10 mayalso impact audio quality. Film grain, dust, surface imperfections, andother imaging anomalies not only degrade image quality, but may alsohave an impact on audio quality. For this reason, a watermark that isapplied along edges of print film 10 used by audio tracks canpotentially degrade the encoded audio signal.

A further complication, related to problem (ii) as given above, is thatthere is no pre-determined widthwise placement of frames 12 and analogsound track 18 and DTS sound track 26 for unexposed film. As the film isshipped from the manufacturer, one orientation is more likely than itsopposite; however, either negative or print film may be rewound beforebeing exposed. Therefore, once print film 10 is manufactured, it cannotbe determined in which direction print film 10 will actually be exposed.Thus, it is not known at the time of manufacture whether analog soundtrack 18 and DTS sound track 26 run along the line of perforations 14nearest one edge of print film 10 or the other. As is observable in theplan view of FIG. 1, frames 12 are skewed to one side of print film 10relative to width W, rather than being centered, to accommodate audiosound track 18 and DTS sound track 26.

A practical watermark exposure scheme, then, must address the problemsof uncertain placement of frames 12 relative to length L and width W,which directly affects robustness and straightforward detection, and ofthe need for encoding analog and digital sound tracks 18, 22, 24, and26. That is, the watermarking scheme that is used must address theproblems posed in considerations (i) and (ii) given above. Conventionalapproaches, such as simply applying a watermark pattern from one edge offilm 10 to the other, could yield unsatisfactory results, degradingimage quality, degrading audio quality, and compromising the robustnessneeded. Some improvement over conventional approaches is needed forproviding watermark exposure, particularly for motion picture filmmedia, that offers a good measure of robustness without introducingproblems related to image and audio quality.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method forexposure of a watermark that is particularly suited to thecharacteristics of motion picture film. With this object in mind, thepresent invention provides a method for applying a watermark patternalong a length of photosensitive medium, the photosensitive mediumhaving a frame pitch in the length direction that is the sum of an imageframe height plus an interframe distance, the method comprising:exposing a plurality of watermark tiles contiguously along the length ofphotosensitive medium, wherein each watermark tile in the plurality ofwatermark tiles has a height dimension that is an integral divisor ofthe frame pitch.

It is a feature of the present invention that watermark tiles aredimensioned based on the frame pitch and perforation scheme used for themotion picture film itself, thereby providing increased robustness forextracting the watermark and eliminating a possible cause of imaginganomalies due to frequency effects.

It is an advantage of the method of the present invention that itprovides an exposure scheme for watermarking motion picture film that issuited to how the surface area of motion picture print film is used.Using the method of the present invention, interference of the watermarkpattern with encoded audio sound track quality can be minimized oreliminated.

It is a further advantage of the present invention that it provides aflexible method for integrating a watermark pattern onto motion picturefilm with minimal impact on image quality, where the watermark patternmay be exposed during media manufacturing or at some subsequent time.

It is yet a further advantage of the present invention that it providesa watermarking scheme that can be used with any type of perforatedphotosensitive medium, as well as with any type of photosensitive mediumhaving periodically spaced notches, indentations, or other features.

It is yet a further advantage of the present invention that it allowsapplication of a watermark without requiring knowledge of where imageframes are printed. This allows the method of the present invention tobe applied before, during, or after exposure of the photosensitivemedium to image content.

These and other objects, features, and advantages of the presentinvention will become apparent to those skilled in the art upon areading of the following detailed description when taken in conjunctionwith the drawings wherein there is shown and described an illustrativeembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the present invention, itis believed that the invention will be better understood from thefollowing description when taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a prior art plan view showing a typical arrangement of exposedareas on a motion picture print film;

FIG. 2 is a prior art plan view showing a portion of an image framehaving an overlaid watermark tile pattern;

FIGS. 3 a and 3 b are plan views showing arrangements of watermark tilesthat are dimensionally formed according to the method of the presentinvention; and,

FIG. 4 is a plan view showing the use of guard bands for watermarkexposure according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present description is directed in particular to elements formingpart of, or cooperating more directly with, apparatus in accordance withthe invention. It is to be understood that elements not specificallyshown or described may take various forms well known to those skilled inthe art.

It must be observed that the method of the present invention is directedto a watermarking scheme that is especially well-suited tophotosensitive media used for motion picture imaging. In an alternateembodiment, however, as described subsequently, the method of thepresent invention could be applied to any type of photosensitive mediumthat allows exposure of a watermark and has some arrangement ofperforations or other regularly spaced features.

Watermark Placement—Length Direction

Referring again to FIG. 1 and to problem (i) noted in the backgroundsection hereinabove, the inherent dimensional relationships of a motionpicture film medium are important to emphasize: image frames 12 areprinted at the same size and with consistent frame pitch F along motionpicture print film 10, so that frame pitch F, the sum of the height ofimage frame 12 plus the dimension of interframe space 16, is necessarilythe same for any motion picture film of a specific size and format, suchas for 35 mm film using the well-known Cinemascope format for example.Referring again to FIG. 2, showing the arrangement of watermark tiles 20within image frame 12, it can be appreciated that, unless watermarktiles 20 appear at the same relative position within each successiveimage frame 12, it can be difficult to extract watermark informationfrom a copied motion picture sequence. That is, positional stability ofwatermark tiles, from one frame 12 to the next, is preferred for ease ofwatermark extraction. In addition, it is possible that frequency-relatedimaging anomalies would be visible due to vertical “walking” of thewatermark images if not spatially aligned from one frame 12 to the nextframe 12.

To address this problem, the present invention applies a method forforming a contiguous arrangement of watermark tiles along the length Lof motion picture film 10 or other photosensitive medium and sizing theheight dimension of watermark tiles 20 relative to frame pitch F.Referring now to FIG. 3 a, there is shown a length of motion pictureprint film 10 using watermark tiles 20 having height dimension H equalto perforation pitch P, as is described in the commonly assignedapplication entitled “Method and Apparatus for Watermarking Film” toRoddy et al., U.S. Ser. No. 10/364,488, cited above. This solution worksbecause frame pitch F is conventionally an integer multiple ofperforation pitch P. The more general solution, however, is to requirethat tile height H of the watermark be an integral divisor of framepitch F. When this condition is met, the following relationship issatisfied:F/H=an integer≧1  (eq. 1)

In the example of FIG. 3 a, tile height H equals perforation pitch P, sothat P/H equals 1. Since frame pitch F is itself an integral multiple ofperforation pitch P, the above relationship (eq. 1) is met. FIG. 3 bshows an example of the more general case, where tile height H is notnecessarily equal to perforation pitch P, but an integral number ofwatermark tiles 20 span a single frame pitch F.

As is noted in the background section above, frame pitch F is someintegral multiplier of perforation pitch P for adjacent perforations 14as shown in FIG. 3 a. By satisfying the relationship of height H ofwatermark tile 20 to frame pitch F given above (eq. 1), the method ofthe present invention aligns each watermark tile 20 appropriately withineach image frame 12. This ensures that the display of successive imageframes 12, as projected, will have the same watermark tile 20 position.This allows watermark tiles 20 to be exposed in a contiguous arrangementalong length L of the photosensitive medium. Moreover, the exposure ofwatermark tiles 20 can be performed before, during, or even afterexposure of frame 12 image content. Exposure apparatus for watermarktiles 20 need not be aligned with actual or intended frame 12 placement,since it is only required that the correct dimensions of tile height Hbe maintained. Note that it is not necessary that the frame edge bealigned with an edge of the tile. Meeting the constraint given above(eq. 1) simply provides that the watermark pattern be in the samespatial position within each frame 12. Tile boundaries themselves aredetermined during the extraction process, as part of spatialsynchronization. In practice, of course, perforations 14 provide auseful reference point for accurate spacing during exposure;particularly where the photosensitive medium may be under varyingamounts of tension as it is processed, causing possible stretching ofthe substrate. However, there is no need to align watermark tiles 20with any specific point along perforation 14 as long as the dimensionalrelationship of height H to frame pitch F is maintained.

Watermark Placement—Width Direction

Addressing problem (ii) noted in the background section hereinabove canbe slightly more complex. As noted in the earlier discussion relative toFIG. 1, it is not possible to know exactly where image frames 12 withaccompanying analog sound track 18 will be placed horizontally, that is,widthwise. Any of the following basic approaches can be employed withrespect to the widthwise placement of image frames 12 and accompanyinganalog sound track 18:

-   -   (1) Use the same set of watermark tiles 20 widthwise across the        photosensitive medium, between left and right perforations 14,        without regard to analog sound track 18 placement. Some impact        on audio quality may result, depending on watermark        characteristics.    -   (2) Use a set of watermark tiles 20 that is specifically        designed to have minimal impact on the analog audio signal.    -   (3) Use one or more guard bands 28 as is shown and described        below with reference to FIG. 4.

Referring to FIG. 4, different bands of the photosensitive medium areoutlined for a short segment of the medium. To allow watermark exposure,the present invention applies a contiguous series of watermark patterns,in the form of watermark tiles 20, along a central watermarking band 30.The area of watermarking band 30 is utilized for image frames 12,whether analog audio sound track 18 and DTS sound track 26 are encodedalong perforations 14 to the left or to the right of image frames 12.One or two guard bands 28 may be provided to one or both sides ofwatermarking band 30. In one embodiment, guard bands 28 are treated asperipheral areas that require some marking in order to match thevisibility of patterns within watermarking band 30. Because exposure ofthe watermark pattern will increase the density and thereby change thecolor characteristics of the photosensitive medium, guard bands 28 cansimply provide a uniform density in one embodiment or can be patternedin such a way that guard band 28 has the same visual appearance as doesthe portion of the projected image within watermarking band 30. In thisway, guard band 28 need not carry the watermark, but could simply have auniform exposure or could be exposed with a pattern that adds densitybut is not encoded with a watermark. Experience with pirated copiesshows that some part of the image frame 12 border is typically croppedduring image capture. Thus, it may be of little consequence if someportion of image frame 12 is without an encoded watermark.

As is noted in the background section above, it is not possible todetermine with certainty, at the time of manufacture, the exactorientation of image frames 12 and their accompanying analog audio soundtrack 18 and DTS sound track 26. However, most motion picture film thatis used professionally prints with one orientation, as determined byfilm packaging. The vast majority of manufactured film is exposed in oneorientation; only a minor percentage of this film is likely to beexposed in the alternate orientation, chiefly due to rewinding. Becauseof this, it may be practical for a watermarking scheme to use only asingle guard band 28, exposed on the side of the film that is mostlikely to contain these audio tracks. Then, should the film happen to beexposed in the opposite orientation, some degradation of audio tracksmay occur. The level of degradation in the audio signal will depend oncharacteristics of the watermark used.

In order to minimize its impact on the encoded audio signals, guard band28 may be provided with a pattern that adds density but, because theencoded signal is beneath some threshold density or is repeated in asequence that does not yield an audible frequency, does not add noise ifdetected within audio sound track 18 or DTS sound track 26. In oneembodiment, for example, watermark tiles 20 are exposed only in theyellow dye-producing (that is, blue-sensitive) emulsion layer of printfilm 10. In this case, guard bands 28 are exposed to produce acorrespondingly yellow tint in order to compensate for the color shiftcaused by watermark exposure within watermarking band 30. Alternately, apattern could be applied over one or both guard bands 28. However, sucha pattern should be designed to have minimal impact on the quality ofanalog sound track 18 and DTS sound track 26.

Guard bands 28 would not be required in an embodiment that employswatermarking tiles 20 whose pattern is itself not audibly perceptible ifimaged in the area of analog audio sound track 18 and DTS sound track26. A watermark pattern having minimal effect on the encoded audio couldbe devised. Such a watermark pattern might simply raise DC levels foraudio sensing, for example, using a uniform exposure or a patternedexposure that is not detected by audio sensing components that readanalog audio track 18 and, therefore, does not generate any measurablenoise.

Applying an Audio Watermark

In another embodiment, watermarking tiles 20 could also be used toprovide a watermark to the audio signal that is encoded in analog soundtrack 18 (FIG. 1). A number of techniques have been proposed forproviding an audio watermark for analog audio signals. For example, U.S.Pat. No. 5,379,345, entitled “Method and Apparatus for the Processing ofEncoded Data in Conjunction with an Audio Broadcast” to Greenburgdiscloses combining an encoded digital data stream as a verificationmarking for a broadcast audio signal. U.S. Pat. No. 6,611,607, entitled“Integrating Digital Watermarks in Multimedia Content” to Davis et al.discloses the incorporation of digital watermarks in an audio signal forcontrol of processing and copying functions. With the watermarkingscheme of the present invention, attributes of watermarking tiles 20could be designed to impose a detectable watermark pattern within theaudio signal, when these watermarking tiles 20 are placed along analogaudio sound track 18. In practice, however, this type of watermark couldprove to be easy to remove using filters or other mechanisms in theaudio path. Watermarking tiles 20 designed for encoding the audio signalmay be formed only within guard band 28 or may be formed over someportion or all of the image surface of the photosensitive medium.

As the above description shows, the method of the present inventionapplies a watermarking pattern that is especially well-suited to therequirements of photosensitive media for motion picture imaging. Whilethis method can have significant value for applying a watermark patternonto a motion picture print film, it could also be used for watermarkingnegative film or other imaging media. FIG. 1 and the accompanying textprimarily describe conventional 35 mm motion picture film; however, itcan be readily appreciated that the present invention applies as well tomotion picture photosensitive media of any size or format, such as 16mm, 65 mm, or 70 mm, for example. Of course, different arrangements ofaudio tracks are possible, depending on the applicable standard for atype of film. Other types of information can also be encoded outside thearea of the image frame, such as bar codes, text, or some other opticalencoding. Similar concerns for avoiding watermark interference with orobstruction of the encoded signal would apply for such other encodedinformation.

In a broader context, the method of the present invention could beapplied to photosensitive media used for still imaging. For example, 35mm negative media is also perforated and typically has image framespositioned relative to film perforations. Thus, it may be useful toapply a watermarking pattern to 35 mm still negative film using themethod of the present invention. Some film types may vary theperforation pitch and have unequal spacing between adjacentperforations, such as APS film uses, for example. However, there isregular spacing of perforations over some interval, and a known framepitch, allowing the method of the present invention to be used with APSor other still films with unequal perforation-to-perforation distances.Alternately, physical features of the photosensitive medium other thanperforations could be used, such as notches or indentations in the filmedge, for example. This would allow the method of the present inventionto be usable with a number of different types of consumer still imagingfilm media, as well as microfilm, photosensitive paper, and otherproducts.

A film manufacturer would apply the watermarking method of the presentinvention as a pre-exposure technique, prior to packaging thephotosensitive medium for shipment. However, watermark exposure couldalternately be performed by a studio before the negative film is exposedin the camera or by a lab, prior to printing a print film. It must beemphasized that the method of the present invention need not beconstrained to pre-exposure. For example, the watermarking tiles couldbe exposed onto a print film during (substantially simultaneously) oreven after exposure to the image content of a frame.

A camera itself could even be provided with an exposure mechanism forencoding a watermark pattern during a film shoot. The same type ofcomponents currently used for in-camera date/time stamp exposure couldbe employed to form a watermark pattern, with the necessary adaptationfor exposure, resolution, and other characteristics suitable towatermark application. For example, a contiguous watermark pattern couldbe exposed within the camera using an arrangement of light-emittingdevices such as LEDs in the film path, controlled by camera logiccircuitry, using techniques well known to those skilled in the art ofcamera design.

Those skilled in the art of film manufacture and handling will recognizethat the method of the present invention may require that watermarkexposure apparatus have periodic synchronization with perforations,since film can stretch with use or stretch due to its winding locationon a roll. It may, however, be more practical to expose the watermarkpattern of the present invention even before perforating the filmmedium, provided that stretching tolerances during subsequent filmprocessing are within an acceptable range.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention. For example, the method of the present invention couldbe used in conjunction with any number of prior art techniques thatapply a watermark pattern to motion picture content. The watermarkpattern, encoded message, or message carrier signal could be changedover a length of motion picture film, using techniques known to thoseskilled in the art.

As is emphasized in the background description given above, prior artsolutions for applying a tiled watermark prove inadequate to meetcertain inherent requirements of motion picture media exposure, largelydue to the arrangement in which images are formed onto the motionpicture medium and due to overall considerations of the use of thesurface area or real-estate of the motion picture medium, particularlywith respect to added analog and digital audio encoding. While partialsolutions have been proposed and may be usable where the exact positionof frame 12 is known, the solution of the present invention provides amore complete solution with respect to these inherent problems of motionpicture media use. With the solution of the present invention, awatermarking arrangement can be obtained that is well suited for a rangeof media types, including motion picture media as well as other types ofstill imaging film and paper, and that can be applied as a pre-exposuremarking or applied during or after exposure to image content.

Thus, what is provided is a method for exposure of a watermark patternonto a length of photosensitive media, such as motion picture film.

PARTS LIST

-   10 Print film-   12 Image frame-   14 Perforations-   16 Interframe space-   18 Analog audio sound track-   20 Watermark tiles-   22, 22′ Digital sound track-   24, 24′ Digital sound track-   26. DTS (Digital Theater Systems) soundtrack-   28. Guard bands-   30. Watermarking band-   F Frame pitch-   H Tile height-   L Length-   P Perforation pitch-   W Width

1. A method for applying a watermark pattern along a length ofphotosensitive medium, the photosensitive medium having a frame pitch inthe length direction that is substantially the sum of an image frameheight plus an interframe distance, the method comprising: (a) exposinga plurality of watermark tiles substantially contiguously along at leasta portion of the length of photosensitive medium; wherein each watermarktile in the plurality of watermark tiles has a height dimension that issubstantially an integral divisor of the frame pitch.
 2. A method forapplying a watermark pattern according to claim 1 wherein said framepitch is an integral multiple of a perforation pitch for thephotosensitive medium.
 3. A method for applying a watermark patternaccording to claim 1 wherein said frame pitch equals four perforationpitches.
 4. A method for applying a watermark pattern according to claim1 wherein the step of exposing a plurality of watermark tiles isperformed as a manufacturing step for the photosensitive medium.
 5. Amethod for applying a watermark pattern according to claim 1 wherein thestep of exposing a plurality of watermark tiles is performed prior toexposure of the photosensitive medium to image content.
 6. A method forapplying a watermark pattern according to claim 1 wherein the step ofexposing a plurality of watermark tiles is performed following exposureof the photosensitive medium to image content.
 7. A method for applyinga watermark pattern according to claim 1 wherein the photosensitivemedium is a motion picture negative.
 8. A method for applying awatermark pattern according to claim 1 wherein the photosensitive mediumis a motion picture print film.
 9. A method for applying a watermarkpattern according to claim 1 wherein the photosensitive medium is amicrofilm medium.
 10. A method for applying a watermark patternaccording to claim 1 wherein the photosensitive medium is a stillimaging medium.
 11. A method for applying a watermark pattern accordingto claim 1 further comprising the step of synchronizing a watermarkexposure apparatus to a perforation location.
 12. A method for applyinga watermark pattern according to claim 1 wherein said watermark tilesare exposed in at least one color plane.
 13. A method for applying awatermark pattern according to claim 1 wherein the step of exposing aplurality of watermark tiles is performed within a camera.
 14. A methodfor applying a watermark pattern according to claim 1 wherein at leastone of said watermark tiles provides an audio encoded signal.
 15. Amethod for applying a watermark pattern according to claim 1 wherein thestep of exposing a plurality of watermark tiles is performedsubstantially simultaneously with the exposure of the photosensitivemedium to image content. 16-39. (canceled)
 40. A method for applying awatermark pattern along a length of photosensitive medium, thephotosensitive medium having a frame pitch in the length direction thatis substantially the sum of an image frame height plus an interframedistance, the method comprising: (a) exposing a plurality of watermarktiles substantially contiguously in a watermark band along at least aportion of the length of photosensitive medium, wherein each watermarktile in the plurality of watermark tiles has a height dimension that issubstantially an integral divisor of the frame pitch; and, (b) exposinga guard band along at least one side of the watermark band.
 41. A methodfor applying a watermark pattern according to claim 40 wherein saidguard band comprises a uniform exposure.
 42. A method for applying awatermark pattern according to claim 40 wherein said guard bandcomprises a watermark encoding.
 43. A method for applying a watermarkpattern according to claim 42 wherein said watermark encoding providesan audio signal watermarking.
 44. A method for applying a watermarkpattern according to claim 40 wherein said guard band comprises apattern for matching the visual density of said watermark band.
 45. Amethod for applying a watermark pattern according to claim 40 whereinsaid watermark band is in only one color plane.
 46. A method forapplying a watermark pattern according to claim 40 wherein saidwatermark band is in only two color planes.
 47. A method for applying awatermark pattern according to claim 44 wherein said pattern lies belowa detection threshold for providing a perceptible audio signal.
 48. Amethod for applying a watermark pattern according to claim 40 whereinthe step of exposing a plurality of watermark tiles is performed as amanufacturing step for the photosensitive medium.
 49. A method forapplying a watermark pattern according to claim 40 wherein the step ofexposing a plurality of watermark tiles is performed prior to exposureof the photosensitive medium to image content.
 50. A method for applyinga watermark pattern according to claim 40 wherein the step of exposing aplurality of watermark tiles is performed following exposure of thephotosensitive medium to image content.
 51. A method for applying awatermark pattern according to claim 40 wherein the step of exposing aplurality of watermark tiles is performed substantially simultaneouslywith the exposure of the photosensitive medium to image content.
 52. Amethod for applying a watermark pattern according to claim 40 whereinthe photosensitive medium is a motion picture negative.
 53. A method forapplying a watermark pattern according to claim 40 wherein thephotosensitive medium is a motion picture print film.
 54. A method forapplying a watermark pattern according to claim 40 wherein thephotosensitive medium is a microfilm medium.
 55. A method for applying awatermark pattern according to claim 40 wherein the photosensitivemedium is a still imaging medium. 56-65. (canceled)